Satellite broadcasting systems transmit digital music and other information from an uplink station to one or more mobile receivers. Satellite broadcasting systems typically include a plurality of satellites and terrestrial repeaters operating in a broadcast mode. The satellites are typically geo-stationary, and are located over a desired geographical coverage area. The terrestrial repeaters typically operate in dense urban areas, where the direct line of sight (LOS) between the satellites and the mobile receiver can be blocked due to the angle of elevation and shadowing by tall buildings.
Orthogonal frequency division multiplexing (OFDM) techniques have been proposed for use in such satellite broadcasting systems and other wireless networks. In an OFDM communication system, the digital signal is modulated to a plurality of small sub-carrier frequencies that are then transmitted in parallel. It has been found that OFDM communication systems do not require complex equalizers, even at high data rates and under multipath propagation conditions. Among other benefits, OFDM communication systems provide a guard interval that absorbs the multipath distortion into the guard interval duration. As long as the arrival times of the multipath signals differ from one another by less than the guard interval, an equalizer is not necessary.
OFDM communication systems are especially sensitive to clipping. In an OFDM communication system, automatic gain control (AGC) mechanisms are an important part of the system design. The automatic gain control mechanisms must provide an analog-to-digital converter with sufficient signal resolution capabilities so that the received analog signal can be represented with sufficient integrity in the digital domain. Designing an effective automatic gain control circuit for use in the environment of an OFDM communication system, however, is challenging due to the large range of channel gains that occur due to long-term or shadow fading. In addition, OFDM receivers often suddenly receive or lose a line-of-sight signal while turning a comer (causing a sudden power increase or decrease on the order of 20 dB). In addition, OFDM receivers often encounter significant interference from the terrestrial repeaters of competitors. Finally, temperature and device variations provide additional challenges.
A need exists for an automatic gain control technique that maintains the received signal quality after analog-to-digital conversion. A further need exists for an automatic gain control technique that performs automatic gain control that maintains linearity throughout the receiver components. Yet another need exists for an automatic gain control technique that performs automatic gain control that reduces clipping and underflow in the analog-to-digital converter.